Science Questions

How fast should I set the fan to defrost the windscreen?

Sun, 10th Feb 2008

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Jim Cooper, Arkensaw asked:

In Arkensaw it occasionally gets very cold and I have to defrost the windshield of my truck before I drive to work in the morning. I wonder what speed I should set the fan on to get the windshield to defrost the fastest because Iíve noticed that if I breathe on my hand with my mouth wide open my breath feels warm. If I exhale through pursed lips the same breath feels cold. Why is that and what should I do with my fan to achieve my fastest defrosting?


The thing thatís going on when you blow on your hand is you tend to blow on it from far away.  Especially if youíre blowing with pursed lips.  If you breathe slowly, especially if youíre close, then all the air to your hand is the nice warm air from your lungs.  When youíre blowing on them with pursed lips, partly itís going to be further away so the air will be cooled down a bit more.  Also, as youíre farther away it will tend to catch cold air from the surroundings and drag that along with it.  Youíre not just blowing along through your lips thereís also a load of cold air mixed in.  Also, the faster the airís moving the more youíll tend to evaporate moisture from your skin which will cool you down as well.  I think if youíre blowing with a big fan very close to your windscreen which is the way it works the higher the fan is the more it will defrost your window.


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